Crosslinked polymer microparticles otherwise known as polymer microgels have been used in the coating industry to produce high solids coating compositions and/or to improve the rheological and other properties of water-based and solvent type coating compositions.
The polymer microgels may be produced by the emulsion polymerization of a monomer mixture containing a polyfunctional monomer having at least two ethylenic unsaturation in the molecule. The resulting synthetic latex may be incorporated into a water-base paint as such. For use in solvent type paints, the microgels in the emulsion may be isolated as discrete particles by spray drying, lyophilization or similar processes, or they may be directly dispersed in conventional organic solvents by solvent substitution. When used in a coating composition, the polymer microgels produced by the emulsion polymerization method tend to adversely affect the durability, particularly the water resistance of the resulting coating films owing to the hydrophilic nature of the emulsifier used in the polymerization.
U.S. Pat. No. 4,477,536 to Howard J. Wright et al. discloses the use of polymer microparticles having polar and/or ionic groups such as carboxyl group on their surfaces in two coat-one bake coating systems. The polymer microparticles are produced by the emulsion polymerization method and the carboxyl groups which induce the electrostatic stabilization in the aqueous medium may be introduced, for example, by using as an emulsifier an unsaturated, water-soluble polyester copolymerizable with the monomers constituting the polymer microgels. Alternatively, polymer microparticles having carboxyl groups on their surfaces may be produced by emulsion polymerizing an acrylic or methacrylic acid-containing monomer mixture. In either case, the emulsifier or monomer having polar groups are localized on the particle surface while leaving the nonpolar components in the particle core. The polymer microparticles having polar and/or ionic groups thus produced are also disadvantageous in that they adversely affect the water-resistance of the coating films when incorporated therein.